The first version of this project used 4 D sized alkaline cells - these lasted a long long time.... Ultimately, I ended up using a small 6V sealed lead acid battery for rechargeable goodness.
The second picture shows the plastic spacers used to interface with the boom box speaker's mount. Also notice the zip tie holding the blue cat5 cable to the circuit board to serve as strain relief in the event the cord should ever get tugged on. This turns out to be absolutely necessary.
The circuit board is mounted to the speaker on one of the machine screws.